Molten iron troughs of the type employed in blast furnace systems generally comprise a refractory substance such as silicon carbide, silicon nitride, carbon, aluminum oxide or silicon oxide which may be blended and compounded with an organic binding agent such as tar or pitch.
Silicon carbide is incorporated as an additional material. This additive serves a variety of useful functions. It accelerates the sintering of the silicon carbide or carbon and thereby retards the formation of pores in the molded trough. Additionally, it extends the useful life of the trough by increasing its strength as well as its resistance to oxidation and corrosion.
Since metallic silicon is expensive, the thus produced silicon nitride is also expensive. The art, therefore, has long sought an acceptable substitute.
One such substitute is ferrosilicon nitride. It is relatively inexpensive. Additionally, the non-nitrogenized ferrosilicon in the product also accelerates the sintering of the trough material. For these reasons, the product has found some acceptance as a substitute for silicon nitride.
The product is not without its disadvantages, however. Tar or pitch which are often used as binders emit offensive odors when exposed to high temperatures. Such replacements as have been proposed have involved the use of volatile organic solvents which are themselves an environmental and health hazzard as well as a potential source of fire or explosions.
In the circumstance the art has sought to replace the organic binders with a combination of water soluble binders and clay both of which are inexpensive and do not adversely affect the environment.
The actual use of clay containing materials, however, meets with certain difficulties.
Ferrosilicon nitride contains a certain amount of impurities including metallic iron. Other characteristic impurities include calcium and aluminum nitrides. These products react with water in the binding agent to produce hydroxides. The reactions deplete the water content of the molded trough in two ways. One is by the reaction itself. The other is by evaporation since the reaction is strongly exothermic. This loss of water results in the production of unsatisfactory molds. The water must be replaced and this has proved to be a source of difficulty. The total amount of impurities is normally up to about 3% by weight and these may include, in addition to those mentioned above, phosphorous, manganese, magnesium and carbon.
Calcium and aluminum nitride are produced during nitrogenization of the starting ferrosilicon which normally contains calcium and aluminum metals as impurities.
The metallic iron causes a similar problem. It is not easily oxidized nitrogenizing metallic silicon at an elevated temperature in nitrogen.
Experience has shown that the problems cannot be solved simply by using an excess of water in the preparation of the troughs or molds. The reason is that the time the refractory compound-clay-binder-water blend will be stored before use varies, and because the optimum amount of water which should be present varies from blend to blend. Accordingly a control which permits the production of uniform troughs with a proper amount of excess water for each trough is difficult to devise.